1. Field of Invention
The present invention relates to optics and more particularly to a focus head for controlling a beam of collimated radiation.
2. Description of the Prior Art
The control of focus and spatial location of radiation such as laser beams containing high power has been found to be very difficult. The prior art includes some apparatus useful in this regard, however, much of the existing equipment has limited function which is inadequate in applications such as materials processing. Almost invariably the prior art apparatus is based on combinations of flat and spherical reflecting surfaces which require that the focusing section be onaxis, a term used herein to describe a mirror oriented to receive incident energy which appears to originate on or close to the line of symmetry for the mirror surface. Spherical mirrors can be heavy but more importantly, they invariably introduce aberrations due to their geometric shape and the manner in which they reflect light. Further, in order to avoid even more severe offaxis aberrations, the axis of the incident radiation and the axis of the reflected radiation form a small included angle and consequently the flat turning mirror has to be positioned at a relatively large separation distance from the spherical mirror in order for the beam to clear itself. Therefore, a focus head based on spherical optics tends to be large and cumbersome because of the large separation distance between the mirrors. Further, the standoff -- the distance between the focus head and the work surface -- is usually kept as small as possible in order to minimize the size of the system. To facilitate the use of lasers in applications such as commercial material processing on a practical basis, a versatile focus head which can be packaged in a practical structure and is capable of providing good optical control to a beam of laser radiation at a relatively long standoff is required.